Extract superclass LevelScreen from TestLevelScreen

It's better to do this sooner rather than later, because we'll need to
do this eventually for multiple levels.
Merge pull request #14 from SweatyReptile/levelscreen_superclass

Extract superclass LevelScreen from TestLevelScreen
Extract body heights

Create a method to determine the height of a body based on its highest
and lowest x coordinate value, where the lowest is at the origin
(Assumes fixed rotation)

Use method to extract height of standing and ducking bodies, which
should be consistent for all variants

[[Warning: Method untested]]
Fix errors made when extracting super class LevelScreen
Create option to switch body from the top

If fromTop is set to true, bodies will treat the top left corner as an
origin when switching, rather than the bottom left.

Body switching from the top is implemented where the legs disappear upon
taking flight. This is so that the legs go up into the body, rather than
the body moving down suddenly (causing a jerky flight).

[[WARNING: Untested!]]
Merge branch 'develop' of https://github.com/SweatyReptile/loser.git into develop
Fix failing body switch

switchBody() was passing oldBody into setTransform(), where it should
have been passing in newBody
Fix pencil body

This was the original reason for
https://github.com/SweatyReptile/loser/pull/17, and it's also the reason
it wasn't used.
Adjust jump height and allow jump-cutting

The initial jump velocity is heightened because the ratio of full jump
to cut-jump wasn't very fun.

Implements https://github.com/SweatyReptile/loser/issues/15
Merge branch 'develop' of https://github.com/SweatyReptile/loser.git into develop
Update README.md with new image
Fix oddity with TestLevelScreen constructor
Rename physWorld -> world to match the name in other places
Create class ScrollingLevelScreen
Set up world just like in TestLevelScreen, for test purposes
Implement basic, jerky scrolling
Refactor method for setting camera position
Camera doesn't scroll unless it's within the bounds of a level

The camera should only scroll when its left edge is within the left
bound of the level (level0), and when its right edge is within the right
edge of the level (levelEnd). 

I tried other methods of representing the level's size, like levelCenter
+ levelWidth, and level0 + levelWidth, but this way seems to be the most
intuitive for use.
Rename ScrollingLevelScreen -> TestScrollingLevelScreen

in preparation of extracting an abstract class ScrollingLevelScreen
Extract super class ScrollingLevelScreen